This application is related to Japanese Patent Application No. 2000-163899 filed on Jun. 1, 2000, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an imaging system in a medical field, particularly to an imaging system which can construct an image from transverse images (tomograms) taken by a CT scanner about an imaging object having a periodically changing shape like a heart.
2. Description of the Related Art
An X-ray computer tomography system (hereinafter referred to as a CT scanner) has been used for taking medical images in a medical field.
The CT scanner is a system which irradiates a diagnosing subject with X-ray to collect projection data of the X-ray transmitted through the diagnosing subject. On the basis of the projection data, an image processing is carried out to reconstruct a transverse image (also referred to as a tomogram) of visceral organs of the diagnosing subject.
Furthermore, by processing a number of the reconstructed transverse images so as to be arranged in the direction perpendicular to the transverse plane at uniform intervals to form a stack, a cross sectional image (herein after referred to as an MPR (multi-planer reconstruction) image) is reconstructed. With three-dimensional information further collected on the basis of the MPR image, production of a three-dimensional image of the diagnosing subject is also carried out.
For producing such a three-dimensional image, a large amount of the transverse images must be collected with a high speed and high precision. For this purpose, a helical type CT scanner (hereinafter referred to as a helical scanning CT) has been recently used which collects data of a helical cross sectional plane.
The helical scanning CT is used for producing a transverse image or a three-dimensional image used for examining and diagnosing a function of a heart. The image of the heart is obtained as being in a diastolic phase or systolic phase. However, a heart repeats a diastole and systole with a certain personally different characteristic period (about 60 beats/minute). Therefore, no plurality of transverse images can be obtained about the heart in the diastolic phase, for example, only at one time scanning.
Thus, in general, an electrocardiograph is provided to select the transverse images supposed to be those in the diastolic or systolic phase in synchronism with data signal obtained from the electrocardiograph. From thus selected transverse images, there are produced the MPR images or the three-dimensional images in the diastolic or systolic phase.
FIG. 1 is a block diagram showing an outline of a configuration of a related synchronization system of an electrocardiograph with a helical scanning CT system.
The synchronization system comprises a helical scanning CT 130, an electrocardiograph 141 outputting data of an electrocardiogram of a diagnosing subject, an electrocardiogram synchronization unit 140 analyzing electrocardiogram data of a diagnosing subject to transmit a synchronization signal to the helical scanning CT, and a computer carrying out an image processing.
In addition, the computer comprises a control unit 100 comprising a CPU and associated units, a storage unit 110, a displaying unit 120, and an inputting unit 121.
In the storage unit 110, there are stored collected signals 111, transverse images 112, MPR images 113, and three-dimensional images 114. The collected signals 111 are those of projection data obtained by the helical scanning CT, transverse images 112 are constructed from the collected signals 111, MPR images 113 are produced from the transverse images 112, and three-dimensional images 114 are produced on the basis of the MPR images 113.
The control unit 100 is provided with various functions such as a signal collection function 101, a transverse image construction function 102, an MPR image production function 103, and a three-dimensional image production function 104. The signal collection function 101 is for driving the helical scanning CT 130 and storing the collected signals 111 of the projection data in the storage unit 110. The transverse image construction function 102 is for constructing the transverse images 112 using the stored collected signals 111. The MPR image production function 103 and a three-dimensional image production function 104 are for producing the MPR images 113 and the three-dimensional images 114 using the transverse images 112 and the MPR images 113, respectively.
In such a related system used for constructing the image of a heart in a diastolic or systolic phase, data collection processing and imaging processing in the control unit 100 were generally carried out by either one of the following.
(1) Processing of driving the helical scanning CT in synchronism with a data signal of the electrocardiograph and obtaining projection data when the heart shows a little motion.
FIG. 2 is a waveform diagram showing a typical data signal of an electrocardiograph.
In general, in a waveform of the data signal of the electrocardiograph, it is well known that a wave form in the range between the T wave and the R wave exhibits a little variation. Here, for allowing the projection data to be obtained within the range, the electrocardiogram synchronization unit 140 transmits a trigger signal to the helical scanning CT 130 at a specified timing. The helical scanning CT 130 receiving the signal carries out a series of processing for obtaining projection data in a followed certain period.
Therefore, in the storage unit 110, only the projection data in the range with a little variation from the T wave to the R wave are stored as the collected signals 111.
After the scanning of the helical scanning CT 130 is over, the control unit 100 constructs a plurality of the transverse images 112 on the basis of the collected signals 111.
Furthermore, there is carried out processing of selecting the transverse images from a plurality of the transverse images 112 in synchronism with the data signal of the electrocardiograph. The selected transverse images 112, for example, those in a period corresponding to the systolic phase of the heart (a specified period after the T wave). On the basis of the selected images 112, processing is further carried out for producing the MPR images 113 or 3 dimensional images 114.
Also about the transverse images 112 in the diastolic phase, processing is carried out for selecting transverse images in a period corresponding to the diastolic phase (a specified period before the R wave).
(2) Processing of obtaining the projection data by the helical scanning CT without in synchronism with a data signal of the electrocardiograph, constructing transverse images on the basis of the obtained data, and selecting images corresponding to those in the diastolic or systolic phase of the heart with a measurer comparing the constructed transverse images and the waveform of the data signal of the electrocardiograph.
Here, the electrocardiogram synchronization unit 140 is not used. Namely, the control unit 100 separately stores the data signal obtained from the electrocardiograph 141 and the projection data obtained from the helical scanning CT 130 in the storage unit 110 as the collected signals 111.
After this, the control unit 100 constructs the axial CT images (transverse images) on the basis of the projection data. The constructed axial CT images include those in both the diastolic and systolic phases.
There, the axial CT images and the waveform of the data signal of the electrocardiograph are displayed on a displaying unit or printed out. From thus displayed or printed out images and waveform, images corresponding to those in the diastolic or systolic phases within the range from the T wave to the R wave are manually selected in order by the measurer with correspondence of each image with time made ascertained.
On the basis of a number of thus manually selected axial CT images, processing is carried out by the control unit 100 for producing the MPR images 113 or 3 dimensional images 114.
The helical scanning CT is a system in which linear movement of a table with a diagnosing subject mounted thereon and rotation of an X-ray tube and detectors are combined to collect the spiral projection data. The path of the X-ray always passes through a certain fixed range within the diagnosing object to provide continuous projection data in the direction of the table movement.
Namely, when transverse images with a certain thickness are constructed with the table position gradually shifted, the time at which the image is obtained becomes a little different depending on the table position. Therefore, each of the transverse images is constructed so as to include projected data obtained at times a little different from each other.
For example, when a whole heart is scanned in 37.5 sec with a helical scanning CT which has an axial resolution (slice thickness) of 2 mm, takes 0.5 sec for one rotation (scanning time =0.5 sec) and reconstructs 10 images per rotation, an image is reconstructed at 0.05 sec time intervals and 750 transverse images are obtained in total.
In general, the period of the diastole and systole of a heart of R-R 60 (60 beats/min) is about 1 sec (60 beats/60 sec). Thus, the 750 transverse images include images for the diastolic phase and those for systolic phase.
Therefore, in the case that MPR images are produced with the 750 transverse images arranged in the direction perpendicular to the transverse plane (axial plane), the MPR images is provided with a wavy outline because of mixed transverse images obtained in both the diastolic and systolic phases. Namely, although an MPR image of a heart would be obtained to have originally a smoothly varying outline when the heart was at rest, the image is obtained to have an indistinct outer shape with the outline thereof having small xe2x80x9cpeaks (maximum peaks)xe2x80x9d and xe2x80x9ctroughs (minimum peaks). This is because the images are taken while the heart is periodically moving.
Therefore, in order to produce the MPR image in the diastolic (or systolic) phase from the transverse images, either one of the above described two kinds of processing was used to select the transverse images in the diastolic (or systolic) phase among a large number of transverse images.
Both of the two kinds of processing, however, requires the measurer to work for visually ascertaining a large number of transverse images and data of electrocardiogram, for which a long period of several days was necessary.
In particular, in the above processing (1), the electrocardiogram synchronization unit 140 was necessary for the synchronization with the data signal of the electrocardiograph, which further required complicated preprocessing for the synchronization including adjustment and parameter setting of the units shown in FIG. 1.
In the processing (2), the work for selecting transverse images required the measurer to have a high expert knowledge and to spend a lot of time. Furthermore, the work of selection was carried out subjectively without flexible applicability.
Accordingly, it is an object of the present invention to provide an imaging system which can construct sharp images with a high speed, high precision and easiness without requiring any electrocardiogram synchronization unit and any image selection processing by a measurer having expert knowledge, even for an imaging object moving periodically such as a heart.
The above object can be accomplished by an imaging system which comprises a CT scanner; a first storage unit storing a plurality of transverse images taken by the CT scanner; an image conversion unit producing a two-dimensional cross sectional image on the basis of the transverse images, for which the transverse images are arranged in a direction perpendicular to a transverse plane; a second storage unit storing the cross sectional image; a peak detection unit extracting an outline of the cross sectional image for detecting peaks or troughs of the outline; a peak interpolation unit carrying out interpolation on the basis of a specified standard for an outline between the peaks or between the troughs detected by the peak detecting unit; and an image reconstruction unit of the cross sectional image stored in the second storage unit using the outline interpolated by the peak interpolation unit.
With the system, the cross sectional image of the diagnosing subject can be constructed with higher speed, higher precision, and more easiness.
In addition, according to the present invention, when an imaging object of the CT scanner is a heart, the image conversion unit produces a cross sectional image of the heart; thereafter, the peak detection unit detects peaks; the peak interpolation unit carries out interpolation for an outline between the detected peaks; and the image reconstruction unit reconstructs the cross sectional image of the heart in the diastolic phase by using the interpolated outline.
Similarly, by using the detected troughs, the cross sectional image of the heart in the systolic phase can be reconstructed.
It is also possible for the system to detect the peaks or troughs with nonuniform intervals. This makes it possible to produce the cross sectional image of the heart in the diastolic or systolic phase with a high speed, high precision, and easiness without visual ascertaining work of the measurer.
In the present invention, the cross sectional image means a so-called MPR (Multi Planar Reconstruction) image. Therefore, the MPR image includes tomograms in all directions which are reconstructed from the projection data obtained by the CT scanner. In the present invention, however, coronal images or sagittal images are to be mainly reconstructed as the MPR images. Particularly in the following embodiments, the coronal image is explained as the MPR image. However, the present invention is not limited to this, but can be applied all kinds of cross sectional images.
In the present invention, the CT scanner may be of any type of scanning method used in the medical field such as T/R method, R/R method, S/R method, N/R method, and helical scanning method. However, since the imaging object of the present invention is a diagnosing subject with a periodical movement, a high speed and high precision CT scanner is required. Thus, it is preferable to use the helical scanning type CT scanner which can finish image-taking of whole diagnosing subject with a high precision while the subject is holding breath.
For the first storage unit and second storage unit, a rewritable storage unit of a semiconductor device (RAM) or a hard disk can be used. In the present invention, a remarkable amount of storage capacity is necessary for both the transverse images stored in the first storage unit and the cross sectional images (MPR images) stored in the second storage unit. Thus, it is preferable to use a mass storage and quick access type storage device such as a hard disk or a magneto-optical disk.
The image conversion unit according to the present invention comprises a control program executing MPR images producing function. Specifically, the unit is a section in which a microcomputer with a CPU executes a specified algorithmic operation on the basis of the control program to thereby output the cross sectional images.
The peak detection unit according to the present invention comprises a control program executing a so-called peak detection function which extracts the outline of the cross sectional image and detects the peaks or troughs of the outline.
The control program is executed with the data of the cross sectional image inputted and data of the positions of the peaks or those of the troughs outputted.
An algorithm for detecting positions of the peaks and the troughs is realized by a difference operation (See xe2x80x9cKagakukeisoku no tame no Hakei Data Shori (Waveform Data Processing in Scientific Measurement)xe2x80x9d, ed. by Shigeo Minami, CQ Publication Co., Ltd., Apr. 30, 1986 (in Japanese)).
The peak interpolation unit according to the present invention comprises a control program executing a function (peak interpolation function) of interpolating between the obtained two peaks (or two troughs). The control program is executed with the peaks and the toughs inputted and an interpolated value between the peaks outputted.
For example, interpolation algorithm is realized by linear interpolation method (See Yasuzo Sudou, xe2x80x9cIgaku ni okeru Sanjigen Gazoshori (Three Dimensional Image Processing in Medical Science)xe2x80x9d, CORONA PUBLISHING CO., LTD., Mar. 10, 1995 (in Japanese)).
The image construction unit according to the present invention comprises a control program executing a function of reconstructing cross sectional images, in which approximately the same algorithm as that used for the image conversion unit can be used. The control program is executed with transverse images corresponding to the interpolated outline inputted and the cross sectional images outputted.
The image conversion unit, peak detection unit, peak interpolation unit, and image construction unit can be realized by single control unit as will be explained later, for example, a personal computer or workstation.
The control program realizing the above described functions can be stored in a fixed storage medium such as a hard disk. In addition to this, the program can be stored in a portable storage medium such as a CD-ROM, MO, or DVD-ROM. Alternatively, the control program can be also stored in a server for being transferred to a personal computer at a remote location through a network.